The invention relates to a process for producing plastic parts, particularly pipes, by extruding a mixture of a polyethylene homopolymer, copolymer or block polymer (PE) and a polymeric plasticizer reactive therewith, as well as small amounts of conventional additives, the moulded or extruded plastic part subsequently undergoing cross-linking and stretching or a cross-linking only. The invention also relates to a master batch for performing the process.
In this generally known process PE and preferably a high density polyethylene (HDPE) is extruded with a reactive polymeric plasticizer, usually with an ethylene vinyl acetate polymer (EVA) and conventional additives, the extruded plastic component then being cross-linked and/or stretched. This process is in particular used for producing pipes, such as water pipes, or shrunk-on sleeves. The cross-linking or stretching takes place to an extent adapted to the intended use, so as to e.g. bring about an adequate shrinking on in the joining regions as a result of heating on joining pipe sections or sleeves.
When performing this process the plastics to be extruded are generally made available as a granular material in a master batch containing the reactive plasticizer and the remaining additives, such as antioxidants, fillers and pigments in a larger proportion of e.g. 10 to 30 and preferably 20% by weight, whilst the remainder of e.g. 70 to 90 and preferably 80% by weight consists of polyethylene. Prior to extrusion, these master batches are then blended with further polyethylene, e.g. in quantities of 10 to 50% by weight.
In hitherto used mixtures of PE and EVA as the reactive plasticizer, the disadvantage has been found that on producing pipes or sleeves there is an irksome splitting off of acetic acid from the EVA under heat action. In addition, the thermal stability of such plastic mixtures is inadequate. It has also been found that the thus produced PE-EVA pipes are relatively rigid and do not have an adequate flexibility.
The problem of the present invention is to improve the aforementioned process or make available master batches making it possible to produce more thermally stable products which, on heating during production, do not split off acetic acid and which in particular have a better flexibility.